Hypothyroidism in Dogs

Hypothyroidism in Dogs

Hypothyroidism is the most common hormone imbalance of dogs. It seems like it would be a straight forward problem: the body does not produce enough thyroid hormone, problems result, a test shows thyroid hormone level is low, the hormone replaced given in pill form, problem solved. Unfortunately, it is not quite so straight-forward in real life. This article attempts to review the issues relevant to this condition and the pitfalls that keep it from being a simple problem.

WHAT IS THE THYROID GLAND AND WHAT DOES IT DO?
The thyroid gland is an H-shape in one's throat. It produces two forms of thyroid hormone: T3 is the active form of the hormone, and T4 is the inactive form created to circulate in the bloodstream. When T4 is absorbed into tissue cells, it is converted into T3. Most of the circulating T4 is carried by blood proteins and is not available for tissue absorption; the portion that is not carried by proteins (the so-called "free T4") is the portion that is able to enter tissues for activation.

Production of T4 is regulated by the pituitary gland at the base of one's brain. (This gland is called the master gland as it regulates hormone production in the adrenal system, the thyroid system, the reproductive system and more). The pituitary produces a substance called TSH, which stands for thyroid stimulating hormone. When T4 levels are dropping, the pituitary gland stimulates the thyroid gland to make and release more T4.
Active thyroid hormone serves as a sort of a volume dial for metabolism. Since virtually every cell in the body can be affected by reduced levels of thyroid hormone it is not surprising that reduced levels of thyroid hormone lead to symptoms in multiple body systems.

WHAT IS HYPOTHYROIDISM?
In short, hypothyroidism is the natural deficiency of thyroid hormone. This deficiency is produced by immune-mediated destruction of the thyroid gland, by natural atrophy of the gland, by dietary iodine deficiency, or as a congenital problem. In the dog, the first two causes listed account for almost all cases.
Hypothyroidism generally develops in middle aged or elderly dogs. Breeds with definite predisposition to develop hypothyroidism include: the Doberman pinscher, the Golden retriever, the Irish Setter, the Great Dane, the Dachshund, Labrador Retriever, and the Boxer.

MANIFESTATIONS OF HYPOTHYROIDISM
Hypothyroidism is a classical disease with a classical collection of clinical signs. One particularly well published survey of 162 confirmed hypothyroid dogs showed the following common findings: 
    88% had some kind of skin abnormality 
    40% had hair loss (often this starts on the tail leading to a "rat tail" appearance or a bald area around the collar    is created) 
   22% had skin infection (often dogs are scaly and smelly due to an excessively oily coat) 
   14% had brittle or dry coats (often the outer hairs break off leaving a short, softer under coat, classically described as a "puppy-like coat") 
   49% were obese 
   48% were described as lethargic or listless at home 
   36% were anemic (had a reduced number of red blood cells due to slowed red blood cell production in the bone marrow.)

One classical finding in hypothyroid dogs is a thickening of some tissues, especially of the face and head. The skin in particular thickens leading to more skin folds and what is classically referred to as a tragic face. This thickening is called myxedema and can occur in some other tissues as well (such as facial nerves - see later).
Cardiovascular Signs - Hypothyroidism interferes with the electrical fibers that more or less provide the wiring for the heart. The rhythmic contractions of heart muscle as normally stimulated by these electrochemical fibers. Abnormal rhythms or slow heart rate occur in as many as 26% of hypothyroid dogs;

Neurologic Signs - According to surveys of confirmed hypothyroid dogs, only about 2% to 4% have nerve problems. There are several syndromes reported.

TESTING FOR HYPOTHYROIDISM
One would think testing for hypothyroidism would be simple: a blood test of the T3 or T4 level could be checked and if it is low, the patient is hypothyroid. Unfortunately, the situation is rarely so simple.

Measurement of T4: The T4 level (also called the total T4 level) is measured commonly and is included in most routine blood panels. It would seem that a low T4 would indicate hypothyroidism and a normal T4 would indicate normal thyroid function. Unfortunately, it isn't that simple. Dogs on certain drugs (most notably phenobarbital, prednisone or other corticosteroids, or trimethoprim sulfa or with illnesses other than thyroid disease often have depressed T4 secretion. These dogs will have low T4's but are not hypothyroid. This means a normal T4 indicates normal thyroid function but a low T4 may or may not indicate hypothyroidism.

TSH Stimulation and Endogenous TSH levels - Before there was readily available equilibrium dialysis free T4, the test that was felt to be the gold standard of reliability was called the TSH stimulation test. In this test, a T4 was measured and then compared with a value drawn 8 hours after administration of an injection of thyroid stimulating hormone. Inability to respond to TSH was considered indicative of hypothyroidism. The problem was expense, the need for 8 hours of hospitalization, and the fact that injectable TSH was frequently off the market. This test is rarely performed nowadays.

TREATMENT OF HYPOTHYROIDISM
At least treatment of hypothyroidism is relatively straight-forward. Hypothyroidism is treated with oral administration of thyroid hormone (T4). Even hypothyroid dogs are perfectly capable of converting T4 to T3. Pills are given usually twice daily to start but may be dropped to once a day after good thyroid control has been achieved.
There are many brands of thyroid supplementation available and prices are somewhat variable depending on the manufacturer.

TREATMENT IS FOR THE LIFE OF THE DOG
Occasionally we are asked if it is reasonable to use dried or powdered thyroid glands of hogs or cattle as a more natural form of treatment. The answer is simply no. These products are not produced with adequate quality control to insure that they contain a reliable amount of thyroid hormone. Each dose may be completely different when such a product is used.

Re-testing later on?
Whenever an animal goes on a medication long term, periodic blood testing is a good idea. In the case of hypothyroidism treatment, it is important to know if the medication dose is too low or too high. Thyroxine (T4) is a very safe medication but if it is not given in adequate doses, obviously the patient will not be adequately treated. If the dose is too high and given for too long a time, excessive water consumption, weight loss, and restlessness can result.

Monitoring with T4
The lowest thyroid level of the day is found by taking the blood sample right before a tablet would normally be given. The highest level of the day is found by taking the blood sample 4-6 hours after it is given. Expect one or both such tests to be periodically recommended. Our hospital recommends some type of blood test monitoring annually.